1. Field of the Invention
The present invention relates to the production and use of organometallic catalysts. In particular, it is directed to bipyridyl metal complexes, to processes for preparing these catalysts and uses thereof.
2. Description of the Related Technology
Hydrogenation of CO2 to form methanol is a novel approach toward storage of a hydrogen source. With hydrogen obtained in a carbon neutral fashion, ideally from photolytic water splitting rather than from fossil fuels, CO2 hydrogenation represents a carbon neutral means of hydrogen storage, because CO2 is absorbed during methanol formation and released during the subsequent methanol combustion.
Catalysts for hydrogenation of CO2 are critical for the success of such a strategy, as the conversion rate and turnover frequency for CO2 hydrogenation is very low without effective catalysts. Hydrogenation catalysts are used in many chemical reactions in addition to CO2 hydrogenation. They generally comprise elements of group VIII of the Periodic Table, e.g. iron, cobalt, nickel, ruthenium, rhodium, palladium, platinum and iridium, as active components. Promoters such as copper, silver, gold, zinc, tin, bismuth or antimony may additionally be present.
In addition, catalysts for oxidation of water to molecular oxygen and hydrogen from photolytic water splitting may play an important role in our nation's long-term sustainable-energy policy. The solar energy may be used as the driving force for oxidation of water to produce oxygen and hydrogen, which may be stored and used as pollution free, alternative energy source.
Homogenous catalysts for both of these processes, hydrogenation and water oxidation, are highly desirable. Homogeneous catalysts tend to have higher efficiencies than heterogeneous catalysts, and they allow for mechanistic studies that can lead to an understanding of intimate mechanistic details. Based on such mechanistic studies, the effectiveness of the catalysts may be further improved.
A catalyst is known that is a half-sandwich ruthenium or iridium complex with 4,4′-dihydroxy-2,2′-bipyridine having the formula [(CnMen)M(H2L1)Cl)]Cl, wherein H2L1 is 4,4′-dihydroxy-2,2′-bipyridine, M is ruthenium when n=6 and M is iridium when n=5 (Himeda, Eur. J. Inorg. Chem., 3927-3941, (2007)). This catalyst has high catalytic activity and is reusable without substantial waste generation because of the automatic tuning of the catalytic activity and water solubility of the catalyst through the acid base equilibrium of the catalyst ligand. Another example of such a homogeneous catalyst is a bipyridyl complex with iridium having the formula [Cp*Ir(bpy)H2O](SO4), where bpy is 2,2′-bipyridine and Cp is a pentamethyl cyclopentadienyl group (US 2010/0016618).
Significant problems still remain in the field of catalytic hydrogenation and water oxidation chemistry. Industrial water splitting as a means of energy storage has yet to be realized, and is potentially a carbon neutral source of hydrogen gas. Hydrogen gas can be a more useful fuel if chemically transformed into other less flammable and more-dense chemicals, and hydrogenation technology provides a means of using hydrogen to create energy dense fuels (for transportation, etc.). However, hydrogenation reactions still often require significant energy input in terms of high pressures and temperatures. Having the ability to catalyze such reactions in aqueous solvents together with high reaction efficiencies would be useful and would save energy.